It is difficult to clean your shop towels as the stains are stubborn and greasy. To wash them out, you need to soak the towels in a bucket that is filled 3/4th with water and two handfuls of powdered Tri Sodium Phosphate. TSP breaks down some of the hardest grease and oil stains and your rags come out clean.
You can also soak them in warm water before washing them in the washing machine with moderately hot water. Do not put these shop towels in the dryer. It is best to dry these towels with air and sun.
Before applying bleach, understand that bleaching softens towels and causes them to wear out faster. Always wash new towels before use. Excess dye, chemicals, etc., can still reside in factory produce towels. You may also use fabric softener on bath towels.